I am an oil whore.
I'll buy whatever is on sale, on clearence, or has a nice MIR.
Having said that, I try to keep each car on the same brand and type of oil for as long as I can.
For example, the '97 Accord is on its third run of M1, and will have at least two more from the stash.
The '99 Accord is on its fourth run of M1, and will do at least four more.
The Subie is on its second run of GC, and I have one more in the stash.
It may then go on PP, since it is thickish (which I think the Sube likes), and I have a lot of it in the stash.
The van gets anything, although I try to avoid mixing brands and types for each fill.